1. Technical Field
The invention relates to optical imaging apparatus, particularly related to polarization imaging apparatus, more specifically related to a high speed polarization imaging system that can measure Stokes components from a sample at different optical wavelengths.
2. Technical Background
The light scattered by a tissue has interacted with the ultrastructure of the tissue, which imprinted some intrinsic properties of the tissue. Tissue ultrastructure extends from membranes to membrane aggregates to collagen fibers to nuclei to cells. Photons are most strongly scattered by those structures whose size matches the photon wavelength. It has been demonstrated that light scattering can provide structural and functional information about the tissue. One important biomedical application of optical imaging and spectroscopy is non-invasive or minimally invasive detection of pre-cancerous and early cancerous changes in human epithelium, such as dysplasia or carcinoma in situ.
In recent years there has been an increasing interest in the propagation of polarized light in randomly scattering media. The investigation of backscattered light is of particular interest since most medical applications aimed at the in-vivo characterization of biological tissue rely on backscattered light. By recording the spatially dependent response of a medium to a polarized point source, one may obtain information about the scattering particles that are not accessible to mere intensity measurements. A diagnostic imaging modality based on near-infrared (NIR) radiation offers several potential advantages over existing radiological techniques. First, the radiation is non-ionizing, and therefore reasonable doses can be repeatedly employed without harm to the patient. Second, optical methods offer the potential to differentiate between soft tissues, due to their different absorption or scatter at NIR wavelengths that are indistinguishable using other modalities. And third, specific absorption by natural chromophores (such as oxy-haemoglobin) allows functional information to be obtained. NIR imaging research has focused on a variety of possible clinical applications.
The fact that the polarization state of the light contains useful information has been shown in many literatures, for example. Rahmann and Canterakis in “Reconstruction of specular surfaces using polarization imaging,” IEEE Computer Society Conference on Computer Vision and Pattern Recognition, vol. 1, pp. 149-155, 2001, describe how the polarization state of light can be used for the reconstruction of specular surfaces to determine the shape of 3-D objects. They use the fact that light reflected by dielectrics and metals becomes partially linearly polarized and that the direction of polarization depends on the orientation of the reflecting surface. Demos and Alfano in “Deep subsurface imaging in tissues using spectral and polarization filtering,” Optics Express, vol. 7, no. 1, pp. 23-28, 2000, demonstrate a technique based on polarization imaging that allows for optical imaging of the surface as well as structures beneath the surface.
There have been several polarization imaging schemes published. In U.S. Pat. No. 6,437,856, inventor Steven Louis Jacques, which issued Aug. 20, 2002, and U.S. Pat. No. 5,847,394, inventors Robert R. Alfano, et al., which issued Dec. 8, 1998, a set of measurements at different polarization orientations are taken to render a new image that is independent of the light reflected from the surface of a tissue sample and that is dependent of the light scattered from deep tissue layers. Especially, a linearly parallel polarized light is used for illumination and two images are acquired, one image selecting linearly parallel (Par) polarized light (i.e., parallel to the light source-tissue-camera plane) and one image selecting linearly perpendicular (Per) polarized light (i.e., perpendicular to the light source-tissue-camera plane). A new image is obtained by (Par−Per) or (Par−Per)/(Par+Per).
Many polarimetric sensing technologies have been developed to capture the Stokes polarization information. People use rotating retarder, rotating polarizer to obtain Stokes parameters from several successive frames of image. In these mechanically-rotation approaches, however, the exact spatial registration between various frames is difficult due to changes of viewing angle and the polarization properties during the period of between successive frames in most target detection applications.
In U.S. Pat. No. 6,798,514, inventor James Maurice Daniels, which issued Sep. 28, 2004, the light passes through two liquid crystal waveplates and a polarizing filter before falling on a light sensitive element device to measure intensity of light in four different polarizations. However, the same difficulty also applied to the approach using liquid crystal for electro-optic polarization modulation, which usually has response time in the order of 100 ms. Another drawback for the liquid crystal approach is that polarization-rotating liquid crystals are not commercially available in many important IR bands.